The field of the invention is motor drives for positioning materials at a work station and, particularly, the movement and positioning of metal during high speed manufacturing processes.
Many articles are formed from sheets of metal which are cut and shaped in high speed automated manufacturing processes. For example, sheet steel may be removed from a roll, cut into pieces, and formed into automobile fenders, refrigerator doors, or the like, at a rate of fifty or sixty per minute. The movement of the metal fabricating material is accomplished using rollers or belts which are driven by conventional a.c. or d.c. rotary electric motors. The rotary motors drive the rollers or belts through a mechanical drive train which may include shafts and gears of various sizes and shapes. The rotary motors are controlled by drive circuits which determine the direction and speed of rotation. Feedback devices such as pulse generators may also be employed with such drive circuits to form closed loop position controls that enable the system to rapidly move the fabricating material and precisely position it in a workstation.
There are a number of difficulties with conventional motor drive systems for high speed metal fabricating processes. First, the rollers or belts employed to propel the fabricating material can mar the surface of the material. This is a major problem when soft metals are used, or when the fabricating material is subjected to high acceleration and deceleration forces. To minimize such problems, the surface area of the driving elements is often increased to distribute the driving force over the surface of the fabricating material. This requires larger, or additional rollers, which in turn increases the size of the drive train from the rotary motors. The mass of the drive train and rollers may exceed that of the driven fabricating material. As a result, the drive motor and its control circuits must be substantially increased in size to meet the desired acceleration and deceleration specifications.
As disclosed in U.S. Pat. No. 4,491,777, most of these difficulties are overcome by the use of a position control system which employs a linear motor to move the sheet material. The sheet material becomes part of the linear motor and is moved with respect to the stationary motor windings with little or no physical contact. The sheet material may thus be moved quickly into a work position without marring its surfaces or without the need for large drive rollers.
Because metal fabricating operations are typically performed when the sheet material is stationary, the sheet feeding control system must continously accelerate and decelerate the material being fed into the work station. On the other hand, the sheet material is typically stored as a large roll which is payed out at a relatively constant rate. As a result, apparatus must be provided for converting the relatively constant feedrate from the roll into the cyclic, stop and go motion into the work station. Traditionally, this conversion has been achieved by providing a loop of sheet material in which material is input at a constant feedrate and removed in a cyclic pattern. The loop is defined by a series of rollers, which on the output side are accelerated and decelerated along with the sheet material. While these need not necessarily be drive rollers if the linear motor drive of the above-cited patent is employed, they still have inertia and can mar the surface of the sheet material.